Various acoustic apparatuses such as speakers or others have been known in the prior art, wherein a piezoelectric oscillation plate, for example, a bimorph cell, is used as a sound source. Although the piezoelectric oscillation plate is excellent particularly in treble characteristics, resonance is liable to occur in the piezoelectric oscillation plate at a certain frequency to cause a peak, whereby the prior art fails to have a flat characteristic over a wide range of frequencies.
Also, there is a problem in that the movement of the piezoelectric oscillation plate is small, whereby it is difficult to sufficiently increase a sound pressure in a bass range by using the piezoelectric oscillation plate.